In a vast majority of motorcycles, steering is accomplished by turning the front wheel relative to the rear wheel. The front wheel is rotatably mounted to a fork, and the fork is rotatably mounted to the frame of the motorcycle. More specifically, the fork includes a fork stem that is rotatably mounted within a head tube on the motorcycle frame. The stem extends all the way through the head tube, and rotation between the stem and the head tube is typically provided by a lower bearing assembly on a lower end of the head tube and an upper bearing assembly on an upper end of the head tube.
The lower bearing assembly commonly includes a radial-thrust bearing (e.g., ball or roller bearing) that prevents upward movement of the stem relative to the head tube. The upper bearing assembly commonly includes a radial-thrust bearing (e.g., ball or roller bearing) that prevents downward movement of the stem relative to the head tube. In this manner, the stem is secured for rotation within the head tube. With proper adjustment of the bearing assemblies, the stem is allowed to rotate smoothly within the head tube to provide a low steering effort (i.e., effort required to turn front wheel).